Hiroki Ogusu

Rattling-Induced Superconductiviy in the β-Pyrochlore Oxides AOs2O6

The superconducting properties of two β-pyrochlore oxides, CsOs 2 O 6 and RbOs 2 O 6 , are studied by thermodynamic and transport measurements using high-quality single crystals. It is shown that the character of superconductivity changes systematically from weak coupling for CsOs 2 O 6 to moderately strong coupling for RbOs 2 O 6 , and finally to extremely strong coupling with BCS-type superconductivity for KOs 2 O 6 , with increasing T c . Strong-coupling correction analyses of the superconducting properties reveal that a low-energy rattling mode of the alkali metal ions is responsible for the mechanism of the superconductivity in each compound. The large enhancement of T c from Cs to K is attributed to the increase in the electron–rattler coupling with decreasing characteristic energy of the rattling and with increasing anharmonicity. The existence of weak anisotropy in the superconducting gap or in the electron–rattler interactions is found for the Cs and Rb compounds.

Superconducting and Structural Transitions in the β-Pyrochlore Oxide KOs2O6 under High Pressure

Rattling-induced superconductivity in the β-pyrochlore oxide KOs 2 O 6 is investigated under high pressure up to 5 GPa. Resistivity measurements in a high-quality single crystal reveal a gradual decrease in the superconducting transition temperature T c from 9.7 K at 1.0 GPa to 6.5 K at 3.5 GPa, followed by a sudden drop to 3.3 K at 3.6 GPa. Powder X-ray diffraction experiments show a structural transition from cubic to monoclinic or triclinic at a similar pressure. The sudden drop in T c is ascribed to this structural transition, by which an enhancement in T c due to a strong electron-rattler interaction present in the low-pressure cubic phase is abrogated as the rattling of the K ion is completely suppressed or weakened in the high-pressure phase of reduced symmetry. In addition, we find two anomalies in the temperature dependence of resistivity in the low-pressure phase, which may be due to subtle changes in rattling vibration.

Understanding of the Temperature–Pressure Phase Diagram of β-Pyrochlore Oxides: A Role of Anhamonicity on Superconductivity

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, \(T_{\text{c}}\), of superconductivity mediated by phonons, via specific heat measurements on β-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that \(T_{\text{c}}\) demonstrates unimodal dependence on pressure, where \(T_{\text{c}}\) increases more than three times by pressure. On the other hand, an electron–phonon coupling constant, λ, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, ωlog, is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of ωlog means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pres...

Superconductivity and Rattling under High Pressure in the β-Pyrochlore Oxide RbOs2O6

Rattling-induced superconductivity in the β-pyrochlore oxide RbOs 2 O 6 is investigated under high pressures up to 6 GPa. Resistivity measurements in a high-quality single crystal show that the superconducting transition temperature T c increases gradually from 6.3 K at ambient pressure to 8.8 K at 3.5 GPa, surprisingly remains almost constant at 8.8±0.1 K in a wide pressure range between 3.5 ( P o ) and 4.8 GPa, and suddenly drops to 6.3 K at P s = 4.9 GPa, followed by a gradual decrease with further pressure increase. Two anomalies in the temperature dependence of the normal-state resistivity are observed at P o P s , revealing the presence of two high-pressure phases corresponding to the changes in T c . The rattling of the Rb ion inside a cage made of Os and O atoms may be slightly and seriously modified in these high-pressure phases that probably have cages of reduced symmetry, respectively, so that electron–rattler interactions that govern the superconducting and transport properti...

Pressure Dependence of Upper Critical Field in β-Pyrochlore Oxides

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs 2 O 6 ( T c =6.3 K) in magnetic field under high pressure. We investigated the upper critical field H c2 at several pressures and found that the slope of H c2 at T c monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m * is enhanced by the pressure. Moreover, the jump of the specific heat at T c is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron–phonon coupling and consequently enhances T c .

Pressure effect on the superconductivity of ?-pyrochlore oxides

We report the results of the specific heat measurements for ?-pyrochlore oxide superconductor RbOs2O6 with superconducting transition temperature Tc ~ 6.3 K in magnetic field and under pressure. We find that Tc rises up to 9.3 K by applying the pressure concomitant with an increase of an initial slope of the upper critical field Hc2 and a jump of the specific heat at Tc. From the analysis of our data, the effective mass of conduction electrons and the electron-phonon coupling appear to increase under the pressure.

Pressure effect on the superconductivity of β-pyrochlore oxides

We report the results of the specific heat measurements for ?-pyrochlore oxide superconductor RbOs2O6 with superconducting transition temperature Tc ~ 6.3 K in magnetic field and under pressure. We find that Tc rises up to 9.3 K by applying the pressure concomitant with an increase of an initial slope of the upper critical field Hc2 and a jump of the specific heat at Tc. From the analysis of our data, the effective mass of conduction electrons and the electron-phonon coupling appear to increase under the pressure.